The present invention relates generally to power control circuits for magnetrons and more particularly to duty cycle power control circuits for magnetrons incorporated in microwave ovens.
Microwave ovens conventionally employ a magnetron for producing microwave energy and a high voltage DC pulsed power supply for the magnetron. A well known high voltage DC power supply circuit is the combination of a high voltage ferro resonant voltage regulating power transformer and a half wave voltage doubler circuit coupling the secondary of the power transformer to the magnetron.
It is a desirable feature in a microwave oven to include means for varying the power level of the microwave energy produced by the magnetron. Conventionally this power level is varied by varying the duty cycle of the magnetron. In this approach full voltage is supplied to the magnetron on an intermittent basis. For example, for a 50% duty cycle voltage supplied to the magnetron and thus microwave power supplied to the food during any instant is either at a maximum or zero, but the average power over a period of time is approximately 50% of the full power. On and Off times ranging from 1 second up to 30 seconds have been employed with on/off times in the 15-30 second range being most common. Typically, an electronic switch such as a triac is employed in the transformer primary circuit to control the duty cycle. In some instances a separate filament transformer is used to energize the magnetron filament. A commonly used more cost effective approach is to provide a filament winding as a secondary on the power transformer.
While use of a filament winding in lieu of a separate filament transformer provides a significant cost advantage, a disadvantage of this approach is that the duty cycle control of the primary winding of the main transformer also duty cycles power to the magnetron filament causing it to undergo considerable variation in operating temperature. Since typically, for domestic microwave ovens duty cycles are on the order of 15 seconds or longer, the filament cools down during the off times and therefore starts from a cold condition which can result in severe moding in the magnetron. In order to reduce the moding associated with starting from a cold condition, the duty cycle may be set to allow for filament pre-heat. Typically, a pre-heat time of at least two seconds is required resulting in a two second loss in cooking time on each cycle.
U.S. Pat. No. 3,392,309 to Hickman addresses the problem of initially turning on an oven with a cold filament. Hickman makes advantageous use of the unique voltage/current characteristic of the magnetron. Specifically, a magnetron draws an insignificant amount of plate current below a threshold anode voltage level, which level is typically approximately 95% of its rated anode operating voltage. When the magnetron anode voltage is below this threshold level very little plate current will flow. Hickman connects a resistor in series with the primary winding. The value of this resistor is selected such that when connected in series with the primary, the anode voltage supplied at the secondary winding is limited to a value slightly less than the threshold level. At this level, however, the output voltage at the filament winding will be sufficient to heat the filament. A timing circuit automatically shunts this resistor after a predetermined warm-up time period has elapsed. Following the warm-up period the full rated anode and filament voltage are applied to the magnetron and the magnetron continues in normal operation.
Hickman is not directed to a duty cycle control arrangement for magnetron, hence there is no attempt to deal with the problem of the cooling of the filament during steady state operation under duty cycle control. In addition, the resistor in the Hickman circuit is an energy dissipating device which adversely affects the operating efficiency of the circuit.
Commonly assigned U.S. Pat. No. 4,318,165 to Kornrumpf et al provides continuous energization of the magnetron filament while duty cycle controlling the magnetron without resort to a separate filament transformer, in a power supply circuit of the high frequency resonant flyback circuit type. Means are provided for supplying filament power directly from the flyback circuit. This resonant flyback circuit is a radical departure in power supply design from the conventional 60 Hz power supply commonly used in domestic microwave ovens involving costly and complex circuitry.
A magnetron control circuit which continuously heats the cathode while duty cycle controlling the magnetron without resort to a separate transformer, without resort to costly energy dissipating circuit elements in the power circuit and which maintains the basic simplicity of the commonly used voltage regulating power transformer and half wave voltage doubler power supply circuit would be highly desirable.
It is therefore an object of this invention to provide an improved duty cycle power control circuit for a magnetron which uses the main power transformer to supply operating voltages to the magnetron filament and to the anode in conventional manner, modified to enable continuous energization of the filament.
It is a further object of this invention to provide a power control circuit of the aforementioned type which requires only a minor relatively inexpensive change to the conventional duty cycle power control circuit for magnetrons incorporated as the microwave energy source in microwave ovens.